Inflammatory Bowel Disease (IBD) is characterized by excess inflammation in the digestive tract resulting in tissue damage. This inflammation has been associated with an imbalance of pro-inflammatory Th17 cells and anti-inflammatory regulatory T cells (Treg). Regulatory T cells are a subpopulation of CD4+ T cells characterized by expression of the transcription factor Foxp3. However, the mechanisms that Tregs use to suppress inflammation remain unclear. Conditional deletion of Stat3 in Tregs results in a fatal inflammatory colitis characterized by an imbalance of Th17 responses but not Th1 and Th2. Based on these results I hypothesize that Stat3 phosphorylation is necessary for Tregs to sense Th17 inflammation. Once activated, Foxp3 can cooperate with Stat3 in the nucleus to coordinate suppression of Th17 responses. To gain a greater understanding of the regulatory network necessary for suppression of Th17 responses, I propose to study the cytokines potentially responsible for Stat3 phosphorylation in Tregs. Using conditional deletion of the receptors for these cytokines from Tregs I will study the role of the individual cytokines on Treg Stat3 phosphorylation and control of Th17 responses. Applying this approach I hope to identify the cytokine primarily responsible for activation of Tregs in the gut. With this knowledge we will proceed to characterize the cellular source of that cytokine and its role in control of Th17 inflammation. By deletion of the cytokine using cell-type specific conditional deletion the cellular players in the regulatory network will be revealed. Finally, to elucidate the transcriptional program responsible for Treg control of Th17 responses I propose to use chromatin immunoprecipitation followed by high-throughput sequencing (ChIP-Seq) to identify target genes of both Foxp3 and Stat3. By combining this data with gene expression profiles as a result of Stat3 deficiency the critical genes necessary for suppression of Th17 responses will be identified and the mechanism of Treg suppression can potentially be elucidated. Together, characterization of the molecular and cellular members of the regulatory network responsible for Treg mediated control of Th17 inflammation will provide functional insights for the choice of biologics and the development of new drugs to modulate the immune response in IBD.